There is known a variety of microwave stripline filters can be used as bandpass filters for a microwave range.
FIG. 1 of the accompanying drawings illustrates an example of such microwave stripline filters of a known type in which it comprises two superimposed dielectric substrates 1a and 1b made of a BaO-TiO.sub.2 or BaO-TiO.sub.2 -rare type dielectric ceramic material having a high dielectric constant and a low loss factor. The dielectric substrates 1a and 1b are provided with ground conductors 2a and 2b on the outer surface and peripheral portion thereof, respectively. On the inner surface of one 1a of the dielectric substrates 1a and 1b are disposed a plurality of strip-shaped resonator conductors 3a which operate as a filter element. Each resonator conductor has one end connected to the ground conductor 2a to form a short-circuit terminal, while the other end of each resonator conductor is not connected to any ground conductor to form an open-circuit terminal. The open-circuit terminals of the strip-shaped resonator conductors 3a are alternately arranged to form an interdigitated-type arrangement. A stripline filter of the above described type is disclosed in Japanese Patent Kokai No. 54-87480.
With such an arrangement in which the resonator conducors 3a having a desired pattern are formed on the inner surface of one of two dielectric substrates 1a and 1b, and sandwiched between said inner surface and the inner surface of the other dielectric substrate, a gap may often be formed partially between the resonator conductors 3a and the inner surface of the other dielectric substrate 1b when the latter is stacked on the former. Therefore, there is disadvantage that electric characteristics between the respective resonator conductors 3a are varied so that the response frequency of the filter may deviated.
In order to improve such disadvantage, there has been proposed another microwave stripline filter in which as illustrated in FIG. 2 two dielectric substrates 4a and 4b are provided with respective resonator conductors 5a and 5b having a substantially same pattern and same dimensions on the respective inner surfaces. The resonator conductors 5a and 5b are bonded together as the dielectric substrates 4a and 4b are stacked to each other (see Japanese Patent Kokai 3-41802).
In the conventional microwave stripline filter in which two dielectric substrates are provided with respective resonator conductors on the respective inner surfaces, and are stacked and bonded to each other, since respective resonator conductors on the respective inner surfaces have same pattern and same dimensions, the stacking of the dielectric substrates should be performed so that the resonator conductors on one dielectric substrate and the resonator conductors on the other dielectric substrate are overlapped without getting out of position. Consequently, the fact is that this assembling process depends upon the manual procedure of person skilled in the art, and thus there are problems in the manufacturing cost and productivity. Further, it is necessary to provide the resonator conductors on the inner surface of each substrate with very restricted tolerance because the resonator conductors should be formed on the inner surface of each dielectric substrate without involving any positional deviation. Even if the resonator conductors are accurately formed on the inner surface of each dielectric substrate with a predetermined position and dimension, and the dielectric substrates are stacked to each other by those skilled in the art, it is substantially difficult to ascertain whether or not the resonator conductors on the both substrates are deviated from each other at stacking them. As a result, the stacked and bonded resonator conductors on the both substrates may be deviated from each other. As shown in FIG. 3, for example, if any deviation occurs in the axial direction of the resonator conductors, there may occur disadvantages that the distance between the adjacent resonator conductors is deviated from a set value and the resonant frequency is varied.
In such stripline filters conventionally proposed, also, when being assembled two dielectric substrates are stacked and bonded to each other by applying adhesive such as cream solder of the like to the whole surface of the resonator conductors on either one of both of the dielectric substrates. As a result, the adhesive 6 can be partly squeezed by and flows out of the resonator conductors. Then, the squeezed bonding agent can adversely affect the electric characteristic of the resonator conductors so that the filter may have deviated resonant frequency, that cannot be corrected in the subsequent stage of fine adjustment of the resonant frequency of the filter, making the yield of manufacture of filters undesireably low.
The use of a relatively large quantity of cream solder and other bonding agents can also deteriorate the Q value of the produced filters because of the low electric conductivity of such agents relative to that of precious material such as silver used for the resonant conductors.
Furthermore, with the conventional stripline filter having two dielectric substrates which are superimposed and bonded with the resonant conductors sandwiched therebetween, it is known that the filter has a resonant frequency characteristic which depends on a relative dielectric constant or a specific inductive capacity of material used as well as the pattern of the resonant conductors. In order to obtain a desired resonant frequency it is of common use that the composition of the substrate material is changed so as to adjust the relative dielectric constant. By using this method it is expected to obtain in design a substrate having a desired relative dielectric constant, but the value of the the relative dielectric constant may be changed after sintering. It is, therefore, the actual circumstances that the adjustment of the composition of the substrate material is repeatedly performed until a desired characteristic is obtained. This leads to a bad productivity.
Furthermore, since the respective dielectric substrates are made of ceramic material having same dielectric constant, the filtering characteristic to be obtained may be substantially determined by the pattern of the resonant conductors which are sandwiched between the dielectric substrates. Therefore, in order to produce a filter having a desired filtering characteristic, it is necessary to design or determine the pattern of resonant conductors whenever the filter is to be manufactured. This also adversely affects the productivity. That is, in the conventional stripline filter of this type, due to the fact that the resonant frequency characteristic to be obtained depends upon mainly the pattern of the resonant conductors and the dielectric constant of the substrate material used, it may be difficult to produce a filter having a relatively exact and desirable characteristic, and thus it is generally required to finely adjust the resonant frequency characteristic after the filter device is assemblied, thereby increasing the manufacturing cost thereof.
It is therefore a first object of the present invention to provide a stripline filter for microwaves capable of overcoming the disadvantages in the conventional filter, having no dispersion in a filtering characteristic and being assembled with a relative large tolerance.
A second object of the present invention is to provide a stripline filter for microwaves wherein it has no dispersion in a filtering characteristic, a good yield can be attained, and any reduction of the Q value can be suppressed.
A third object of the present invention is to provide a stripline filter for microwaves wherein a resonant frequency characteristic can be finely selected and a desired value thereof can be relatively exactly obtained.